In the oil and gas industry, but also in other industries as the chemical or petrochemical industry it is often necessary to transport a multiphase fluid comprising liquid and gas through a conduit. For example, hydrocarbons (crude oil or condensate, sometimes with water) and gas need to be transported from a well through a pipeline to a process facility. In case of offshore oil production crude oil, production water and associated gas are generally simultaneously transported through a subsea pipeline to gas/liquid separating equipment located onshore or on an offshore platform. The pipeline or flowline system may include a riser section.
A particular problem in such operations is the occurrence of plug flow. In plug flow, a batch of one of the phases is formed and transported through the conduit. A batch of liquid is sometimes also referred to as a slug. In an undesirable situation, liquid slugs and gas surges are produced alternatingly through the conduit. Such an alternating pattern of liquid slugs and gas surges presents problems for downstream equipment such as a gas/liquid separator, as it imparts separation efficiency and capacity use of the separator.
Liquid slugs can be formed by operational changes, e.g. the increase of the fluid production during the start-up of a pipeline. Liquid slugs can also be formed due to the geometry of the conduit (“terrain slugs”), or due to an unstable liquid/gas interface (“hydrodynamic slugs”). In an oil/gas riser system to a processing unit, a small liquid plug at the riser foot has a tendency to grow due to the hydrostatic pressure that builds up in the riser pipe, and a volume of gas is formed behind the liquid slug. This phenomenon is also known as “severe slugging”, whereas slugs formed upstream of the riser foot are commonly referred to as transient slugs.
EP-B-767699 and WO 01/34940 both disclose methods of preventing growth of liquid slugs in a stream of multiphase fluid, wherein the multiphase fluid is admitted into a gas/liquid separator having gas and liquid outlet valves, and wherein the valves are operated in response to one or more suitably selected control variables such as the liquid level in the separator, the liquid flow rate, gas flow rate, or the total volumetric flow rate from the separator.
US 2003/0010204 A1 discloses another method of controlling severe slugging in a riser of a pipeline arrangement, wherein also a gas/liquid separator is arranged at the upper end of the riser, and wherein the gas outlet from the separator is controlled in response to a pressure measured at the riser foot.
U.S. Pat. No. 6,286,602 discloses a method for controlling a device for transporting hydrocarbons in the form of a mixture of liquid and gas from a production means through an upward pipe, into which gas is injected at the lower end for lifting the hydrocarbons to a treatment plant. During production the flow is controlled by a controller. The controller compares a parameter which characterizes the start of an interruption in the flow of gaseous hydrocarbons, calculated from time averages of the pressure at the lower end of the pipe with a predetermined value, and manipulates both the gas injection rate and a downstream valve if the predetermined value is exceeded. If the predetermined value is not exceeded, the flowrate of produced hydrocarbons is compared with a target flowrate, and deviations are counteracted by manipulating the gas-injection rate.
In an article “Suppression of slugs in multiphase flow lines by active use of topside choke—Field experience and experimental results”, Proceedings of the 11th International Conference on MULTIPHASE flow, San Remo, Italy, June 2003, by G. Skofteland and J.-M. Godhavn, a multiphase flow control method is disclosed wherein the volumetric flow is stabilized by manipulating a choke at the top side of a riser flowline. The volumetric flow is determined from the pressure difference over the choke, the choke position, and the density of the multiphase fluid which is measured using a gamma densitometer upstream of the choke.
It is an object of the present invention to provide a method for controlling multiphase flow in a flowline, in particular to suppress and control plug flow, which is robust and simple, and which requires a minimum of hardware for its operation.